Transformation behavior of alkali metals in high-alkali coals

Abstract

Burning a coal containing a high content of alkali metals (Na, K) in utility boilers may result in ash-related problems such as slagging, fouling and corrosion on the heat transfer surfaces. This study investigated the transformation behavior of Na in four typical high-alkali coals including one Zhundong (ZD) coal and three Taer (TR) coals during combustion. The Na content in the coal is much higher than the K content, and different high-alkali coals have difference in the aspect of their existing forms. Measurement results of the tested coals showed that most of the Na in the coal is H2O-soluble Na. During combustion, the released alkali metals increase with the elevated temperature. The H2O-soluble Na and NH4Ac-soluble Na in the ash decrease after the coal is burnt. This indicates that the released Na is originated from the H2O-soluble and NH4Ac-soluble Na, with the H2O-soluble Na being the dominant constituent. The investigation also reveals that some of the H2O-soluble Na in the ash might be converted into HCl-soluble Na and insoluble Na through reacting with Si-Al minerals. Furthermore, the low volatility of Na for ZD and TR coals is closely associated with the content of Cl, Si and Al in the coal.